We bought the book The Happiest Baby on the Block when Francesca was about three or four weeks old. It saved my sanity. The short version is that he has 5 S's - swaddling, swinging, shushing, side/stomach-lying, and sucking and goes through how to use each one. While none of it is rocket science, it really helped me to have the book to break it all down for me.

My son had HORRENDOUS all day colic-he was later diagnosed with an allergy to dairy and soy. Approximately 10% of babies are allergic to dairy and of those, 50% are allergic to soy. Once I cut dairy and soy out of my diet (as I was breastfeeding), and cut out formula supplementation (I thought he was crying due to hunger), the crying decreased tremendously. He seriously was crying UP TOP 18 HOURS A DAY at 6 weeks of age and after I removed the dairy/soy he cried 2-3 hours a day. I have spoken to many, many mothers who have removed dairy and have seen huge differences in their child's behaviour. Definitely worth a try.

I second the dairy/soy - I had to cut dairy/soy/eggs out of my diet with DD and she went from screaming crying for HOURS to a happy baby within a few days - it was like a miracle. It can take up to 3 weeks totally clear your system, but usually if it's to the point where the baby is crying like that, you will see a difference after a few days - and dairy is in EVERYthing - it's under stuff as casein and whey, so read labels if you try this out.